Observations at a mountain-top site within the Mexico City basin are used to
characterize ozone production and destruction, nitrogen oxide speciation and
chemistry, and the radical budget, with an emphasis on a stagnant air mass
observed on one afternoon. The observations compare well with the results of
recent photochemical models. An ozone production rate of ~50 ppbv/h
was observed in a stagnant air mass during the afternoon of 12 March 2006,
which is among the highest observed anywhere in the world. Approximately
half of the ozone destruction was due to the oxidation of NO<sub>2</sub>. During
this time period ozone production was VOC-limited, deduced by a comparison
of the radical production rates and the formation rate of NO<sub>x</sub> oxidation
products (NO<sub>z</sub>). For [NO<sub>x</sub>]/[NO<sub>y</sub>] values between 0.2 and
0.8, gas-phase HNO<sub>3</sub> typically accounted for less than 10% of
NO<sub>z</sub> and accumulation-mode particulate nitrate
(NO<sub>3(PM1)</sub><sup>&minus;</sup>) accounted for 20%–70% of NO<sub>z</sub>,
consistent with high ambient NH<sub>3</sub> concentrations. The fraction of
NO<sub>z</sub> accounted for by the sum of HNO<sub>3(g)</sub> and
NO<sub>3(PM1)</sub><sup>&minus;</sup> decreased with photochemical processing. This
decrease is apparent even when dry deposition of HNO<sub>3</sub> is accounted for,
and indicates that HNO<sub>3</sub> formation decreased relative to other NO<sub>x</sub>
"sink" processes during the first 12 h of photochemistry and/or a
significant fraction of the nitrate was associated with the coarse aerosol
size mode. The ozone production efficiency of NO<sub>x</sub> on 11 and 12 March
2006 was approximately 7 on a time scale of one day. A new metric for ozone
production efficiency that relates the dilution-adjusted ozone mixing ratio
to cumulative OH exposure is proposed.